A surface discharge AC plasma display panel (hereinafter referred to simply as a “panel”) is one of typical plasma display panels, and the surface discharge AC panel includes numbers of discharge cells formed between a front substrate and a rear substrate which confronts the front substrate. Both of the substrates are made of glass. On the front substrate, a plurality of display electrodes, each one of which is formed of a pair of a scan electrode and a sustain electrode, are formed in parallel with each other, and a dielectric layer and a protective layer are formed such that they cover the display electrodes. On the rear substrate, a plurality of data electrodes are formed in parallel with each other, and a dielectric layer is formed to cover the data electrodes, on top of that, a plurality of barrier ribs are formed in parallel with the data electrodes. A phosphor layer is formed on the surface of the dielectric layer and on the lateral faces of the barrier ribs.
The front and rear substrates confront each other and are sealed such that the display electrodes intersect with the data electrodes, and the space between the front and rear substrates sealed is filled with a dischargeable gas. This structure allows forming discharge cells at each confronting section between the display electrodes and the data electrodes. In every discharge cell, ultraviolet ray is generated by gas discharge, and the ultraviolet ray excites the phosphors to emit red, green, and blue colors, so that a color display is achieved.
A sub-field method is widely used for driving the panel. According to this method, one field period is divided into a plurality of sub-fields, then a combination of the sub-fields, which are supposed to emit light, allows displaying a grayscale. Each one of the sub-fields has a given brightness weight, and lighting the sub-fields results in a given brightness display in response to the brightness weights. Among the sub-field methods, light emitting of sub-fields not involved in the gray scale display is reduced as much as possible for suppressing the black brightness, thereby increasing a contrast ratio. This driving method is disclosed in, e.g. Unexamined Japanese Patent Publication No. 2000-242224.
Hereinafter, the foregoing driving method is detailed. FIG. 8 shows driving waveforms illustrating a conventional driving method of a panel. Each one of sub-fields has an initializing period, an addressing period, and a sustaining period. During the initializing period, the cells involved are entirely initialized or selectively initialized. To be more specific, the discharge cells involved in displaying a video are entirely initialized for discharging, or only the discharge cells that carried out the sustain discharge at the immediate last sub-field are selected and initialized for discharging. In the driving waveforms shown in FIG. 8, the entire initialization is done during the initializing period of the first sub-field (hereinafter sometimes referred to simply as “SF”), and the selective initialization is done during the initializing periods of the second SF and onward.
First, during the initializing period of the first SF, all the discharge cells are initialized for discharging in order to erase the historical record of wall charges of the respective discharge cells as well as form wall charges necessary for the coming address operation. On top of that, this initialization also generates priming, i.e. generates excited particles which can minimize a discharge delay as well as generate an address discharge in a stable manner. This entire initialization is done this way: Keep all the data electrodes and the sustain electrodes at 0 (zero) volt (grounding potential), then apply a lamp voltage to all the scan electrodes. The lamp voltage moderately increases from voltage Vp lower than the discharge starting voltage to voltage Vr over the discharge starting voltage.
The foregoing preparation allows all the discharge cells to discharge faintly, and the sustain electrodes as well as the data electrodes to store positive wall charges thereon, and the scan electrodes to store negative wall charges thereon. Then keep all the sustain electrodes at voltage Vh, and apply a lamp voltage to all the scan electrodes. This lamp voltage moderately decreases from voltage Vg to voltage Va, so that all the discharge cells faintly discharge for weakening the wall charges stored on the respective electrodes. The entire initialization discussed above allows the voltage in the discharge cells to become close to the discharge starting voltage.
During the addressing period of the first SF, apply scan pulses sequentially to the scan electrodes for scanning the scan electrodes, and apply address pulses, corresponding to the video signals to be displayed, to the data electrodes, thereby generating address discharge between the scan electrodes and the data electrodes in the discharge cells to be displayed, namely, display cells, for forming wall charges selectively. During the sustaining period following the addressing period, apply sustain pulses a given times in response to the brightness weight between the scan electrodes and the sustain electrodes, thereby generating sustain discharge in the discharge cells, in which wall charges have been formed with the address discharge, for emitting light. This light emission allows displaying a video.
During the initializing period of the second SF, keep all the sustain electrodes at voltage Vh, and all the data electrodes at 0 (zero) volt. Then apply a lamp voltage to all the scan electrodes. This lamp voltage moderately lowers from voltage Vb to voltage Va. While the lamp voltage lowers, the discharge cells, which have carried out sustain discharge during the immediate last sustaining period, i.e. the sustaining period of the first SF, faintly discharge for adjusting the wall charges formed on the respective electrodes. The voltage in the discharge cells thus becomes close to the discharge starting voltage. On the other hand, the discharge cells, which have not carried out the address discharge and the sustain discharge during the first SF, do not discharge even faintly during the initializing period of the second SF, so that the wall charges are kept as they are at the time when the initializing period of the first SF ends.
During the addressing period and the sustaining period of the second SF, apply a driving waveform similar to that of the first SF to the respective electrodes, thereby generating sustain discharge in the discharge cells corresponding to video signals. During the third SF and onward to the final SF, apply a driving waveform similar to that of the second SF to the respective electrodes, thereby displaying a video. The brightness weights in the respective sub-fields are set, for instance, to increase step by step from the first SF to the final SF.
In the case of displaying uniformly a video of lower part of grayscale on the entire screen by using the conventional driving method discussed above, the following method is taken as an instance: When the sustain discharge is carried out only in the first SF where the lowest part of grayscale takes place, a dark area sometimes occurs in a part of the screen, and the dark area is a belt-like shape and has a lower brightness than other areas. In general, the panel is placed for displaying videos such that the scan electrodes and the sustain electrodes arranged horizontally, and the data electrodes are arranged vertically. In the case of using the panel driven by a single scanning method, a horizontal dark belt can be seen sometimes at the lower part of the screen. In the case of using the panel driven by a double scanning method, the horizontal dark belts sometimes can be seen at the center and at the lower part of the screen.
The panel driven by the single scanning method scans every scan electrodes sequentially from the top during the addressing period, while the panel driven by the double scanning method scans the scan electrodes in the upper half area and those in the lower half area respectively and sequentially from the top of each area with the timings nearly equal to each other. FIG. 8 shows the driving waveforms of the panel driven by the single scanning method.
Since the conventional driving method discussed above sometimes invites the foregoing dark belt, it is difficult to display uniformly the video of a lower part of gray scale on the screen. The display quality thus becomes poor. In the case of the panel driven by the double scanning method, in particularly, the dark belt occurs at the center of the screen conspicuously, so that the display quality becomes worse.